Water-soluble alkali metal and ammonium salts of copolymers of styrene or olefins with maleic anhydride have been known and used for a variety of purposes for many years. For example, Stump, in U.S. Pat. No. 3,451,890, teaches the use of small amounts of such water-soluble salts to produce improved rosin size compositions for incorporation in wet-laid paper. Farley, in U.S. Pat. No. 3,261,798, employed water-soluble salts of low-molecular-weight copolymers of maleic anhydride with olefins having 6 to 24 carbons to coat or impregnate preformed paper to improve brightness, ink reception and the like.
Alkali metal salts of cross-linked copolymers of styrene or olefins with maleic anhydride are also known and have been used particularly as efficient thickeners for aqueous compositions as shown, for example, by Seymour, U.S. Pat. No. 2,647,886 and Johnson, U.S. Pat. No. 3,393,168. Seymour considered his products to be water-soluble but subsequent studies have shown such cross-linked polymers to be, in fact, water-insoluble although, when lightly cross-linked, highly water-swellable and thus, as taught by Johnson, "forming a dilute gel or dispersion in aqueous medium."
Recently, in U.S. Pat. No. 3,669,103, it has been suggested that highly water-swellable, water-insoluble polymers be incorporated in paper to be used to increase the sorbency of sanitary articles. However, it has been found that the highly hydrophilic alkali metal or ammonium salts of cross-linked maleic anhydride copolymers, such as those taught by Seymour and Johnson, are totally unsuited for incorporation as such into a papermaking furnish since they render the furnish too viscous for proper running on the screens and in addition adversely affect the formation of the paper web and introduce severe drying problems.
It would therefore, be highly desirable, and is an object of the present invention, to have available a method whereby the cross-linked copolymers of styrene or olefins with maleic anhydride can be incorporated in a paper web in a form relatively inert to water and thereafter be converted to the highly water-swellable salt form.